It is known that crystalline solar cells in which monocrystalline silicon or multicrystalline silicon is used as a substrate have energy conversion efficiency higher than that of amorphous solar cells. When light reaches the silicon substrate of solar cells, sunlight divides into light entering inside the substrate and light reflecting on the substrate surface. Only the light entering inside contributes to photovoltaic effects.
In conventional crystalline solar cells, selective etching is given to the substrate surface to provide a so-called texture configuration in which multiple pyramids are continuously formed, thereby having improved energy conversion efficiency. This is a configuration utilizing an optical confinement effect in which even light once reflected on the slant plane of the pyramid can be received on the slant plane of the adjacent pyramid and is allowed to be made incident through refraction, unlike a case where the substrate surface is even.
Such a texture configuration of the substrate surface is in general formed by subjecting a substrate having the (100) orientation to anisotropic etching in an alkaline solution, in a case where the substrate is made from monocrystalline silicon. In addition, where a substrate is made from multicrystalline silicon, the orientations individually vary depending on crystal grains, and anisotropic etching may fail to provide an optimal texture configuration to which pyramids are connected, thereby a machining method, reactive ion etching (RIE), isotropic wet etching and others are adopted for decreasing the surface reflection. In the machining method, the surface of a multicrystalline silicon substrate is mechanically grounded down by a dicer into V-shape grooves and then subjected to chemical etching to smooth the V-shaped slant planes, thereby forming a texture configuration. In the RIE method, gases such as SF6, CF4 and Cl2 are introduced into an etching chamber and glow discharge is allowed from a high-frequency power source to provide etching. Isotropic wet etching is a chemical etching using nitric acid and hydrofluoric acid.
However, in the case of monocrystalline silicon solar cells, monocrystalline silicon itself is very expensive, whereas in the case of multicrystalline silicon substrates, from the necessity of machining individually in a machining method, taking too much time in providing many desired grooves is required, resulting in higher production costs. In the RIE method, introduced gases such as SF6, CF4 and Cl2 into an etching chamber are corrosive, therefore, the device must be composed of anti-corrosive materials, resulting in higher costs. Isotropic wet etching provides a textured substrate surface which is 20%, or still high in the reflectance, inferior in energy conversion efficiency and emits a harmful gas of NOx due to the use of nitric acid in the etching solution. By any of the above methods, a serious problem has not been solved where the energy conversion efficiency of multicrystalline silicon solar cells is lower than that of monocrystalline silicon solar cells.
Therefore, an object of the present invention is to provide a multicrystalline silicon substrate having a lower reflectance at a lower cost.